The present invention relates to an oil burner control system; and more particularly, it is directed to an adapter circuit for retrofitting an existing oil burner control system with a flue damper. Such control systems may be used with an oil burning furnace or other oil burning unit.
Flue damper assemblies, including a rotatable damper plate and a damper operator, together with limit switches which may be cam-operated, are commercially available, and have been used extensively in both gas and oil burning units as a means of saving energy by reducing heat loss through the flue after the burner is shut off and combustion products have been exhausted through the flue. The desirability of flue dampers has increased substantially with fuel shortages and the high cost of fuel, so there is a substantial market for retrofitting existing furnaces and the like, and in such cases, the circuitry which interfaces the damper assembly with the existing thermostat and oil burner control system is referred to as the "oil damper adapter" or, simply, the adapter circuit.
Adapter circuits for oil burner control systems are also commercially available, but present commercially available adapter circuits have one or more disadvantages. For example, some damper adapter systems presently available require that the adapter circuit carry the full electrical load, rather than having the load carried by the oil burner control system which is intended for this purpose. Thus, the adapter circuits presently available require re-wiring of the line voltage hook-up between the oil burner control and the motor and ignition circuits. Another disadvantage of some commercially available damper adapter arrangements is that by incorporating them into an existing system, they interfere with the safety timing on the oil burner control. That is, after the oil burner control system is retrofitted with a flue damper, the timing permitted by the oil burner control to sense a burner flame after power has been applied to the burner motor and ignition is reduced by the time required for the damper to open; and this may result in false lockouts by the oil burner control system. Briefly, the present invention overcomes these disadvantages, and further, adds significant safety features such as an interlock between the damper and the line voltage motor and ignition circuit such that if the oil burner motor and ignition are energized, through fault or tampering by an unskilled person, the damper will be driven to the open position, independent of the operation of the thermostat in the low voltage circuit. Further, an exhaustive and detailed fault analysis of the present system indicates that there is no safety hazard presented with any single fault or with any realistic occurrence of multiple faults in any of the subsystems including the thermostat, the oil burner control circuit, the damper assembly, the damper operator circuit or the oil damper adapter circuit.
The adapter circuit of the present invention includes a source of energy in circuit with the thermostat for energizing the damper operator motor in response to a call for heat to drive the damper plate to the open position prior to energizing the oil burner motor and ignition circuit. A first relay in the adapter circuit includes a pair of contacts for simulating the operation of the thermostat contacts. This relay is not energized until the damper plate has reached the open position; and when the relay is energized, its contacts close to simulate the operation of the thermostat and generate a control signal to energize the burner control circuit. Thus, the burner motor and ignition are energized by the existing oil burner control circuit relay and the safety timing required for sensing the main burner flame is not diminished by the time required for opening the flue. This first relay in the adapter circuit is sometimes referred to as the "simulating" relay because, as indicated, its contacts simulate the operation of the contacts of the thermostat during a call for heat.
A second relay, sometimes referred to as the "safety" relay in the adapter circuit senses whether the burner motor and ignition are energized. When the oil burner motor is de-energized, the safety relay initiates a time delay after which the adapter circuit closes the damper. This safety relay also provides an interlock between the line voltage supplied to the motor and ignition circuit of the conventional oil burner control system and drives the damper to the open position whenever the oil burner motor and ignition circuit is energized independent of the condition of the thermostat in the low voltage circuit. Thus, in the event of a fault or deliberate tampering with the low voltage control circuit in an effort to cause the furnace to come on, the damper will be driven to the open position.
Another advantage of the present invention is that it facilitates retrofitting an existing oil burner control system with a flue damper by having a separate housing for the adapter circuit which is designed to be mounted in the furnace adjacent the existing oil burner control box and without having to cut or rewire any of the existing leads in the oil burner control. Only splices are required for interfacing the oil burner control with the adapter circuit, and the thermostat wires are routed directly to the adapter circuit. Further, connections to the damper operator assembly are facilitated by a cable and polarized end plugs interfacing the adapter circuit with the damper operator, thus avoiding possible mis-wiring or tampering. Other features and advantages, both operational and safety, will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views.